The present invention is directed to a fiber composite component that is at least partially produced by braiding, particularly a compression-tension strut, and to a method of producing such fiber composite components. The invention also includes an aircraft having such a compression-tension strut or fiber composite component.
Fiber composite components are used in aerospace engineering and in automotive engineering, to save weight. For this purpose, construction types consisting of CFRPs (carbon fiber reinforced plastic) have been developed to replace previous metallic components joining elements, transverse links, etc.) with fiber composite components. For example, numerous commercial aircraft currently use metallic rear links for the bearing of landing flaps; however, such metallic links should also be replaced by CFC structures to save weight.
Such a CFC concept may be based, for example, on the use of correspondingly tailored and deposited non-crimp fabrics. Such laminates can be provided, as required, with bores for receiving bearings or bolts.
European Patent Document EP 0 398 841 A1 discloses an accelerating lever which is constructed of several stacked layers in the form of prepreg (pre-impregnated) materials or semi-finished products. The layers have different fiber orientations and essentially extend parallel with respect to a plane of motion.
Furthermore, German Patent Document DE 196 28 388 A1 discloses a compression-tension strut, which is formed by depositing reinforcing fibers (such as glass fibers) corresponding to a defined laying pattern, and fixing by means of embroidering on a nonwoven material (e.g., fleece), with additional Z-axis reinforcements being provided. A disadvantage of such components produced by means of TFP (tailored fiber placement), however, is that only a small component thickness can be obtained, because the embroidering base has negative effects on the mechanical characteristics of the component, particularly due to lateral contractions during tension or compression loads.
In addition, such compression-tension struts may be produced with corresponding eyes, loops, lugs or passage openings, by means of a winding process, for example, for bearing bolts. However, in this case only a single component can be produced, and winding is a high-expenditure process. In addition, when winding at non-zero angles, an inaccurate deposit of the winding layers (overlaps or gaps) may occur which, in turn, has a negative influence on the mechanical characteristics of the component.
In the case of such fiber composite components, an optimal design with respect to the loads to be absorbed (particularly the tensile and compressive forces) generally is problematic.
It is therefore an object of the present invention to create a fiber composite component with optimized load absorption, to replace corresponding metallic components used so far.
Another object of the invention is to provide a method by which such fiber composite components can be produced effectively and therefore cost-effectively.
These and other objects and advantages are achieved by the fiber composite component according to the invention, which consists of a semi-finished fiber composite product, comprising a core element with at least one concavely constructed face that is adjoined by a passage opening. The respectively opposite lateral or longitudinal sides of the core element, as well as the side of the passage opening not bounded by the core element, are enclosed in a loop-type manner by a braided fiber composite element. Finally, for forming the finished fiber composite component, the semi-finished fiber composite product is infiltrated in a known manner by a resin system and hardened.
As used herein, the term “concave” is generally understood to mean a recessed feature; that is, the concavely constructed face may, for example, be curved toward the inside. However, the shape of the face need not necessarily be curved; rather, it may, for example, also have an angular shape. The shape of the passage opening is expediently adapted to the shape of the respective face.
Such a construction provides a fiber composite component with reduced weight in comparison to corresponding metallic components, which is also optimized for absorbing loads, such that the compression forces are absorbed by the core element and the tensile forces are absorbed by the braided fiber composite element arranged in the manner of a loop. By using the braiding technique, particularly the circular braiding technique (one example of which is disclosed in the unpublished German Patent Application DE 10 2004 017 311, whose content is to be part of this specification), the fiber composite component can be produced without any major loss of mechanical characteristics.
The at least one concavely constructed face preferably has a defined radius of curvature which is adjoined by an essentially circularly constructed passage opening preferably in a continuous manner (that is, without gaps or slots), and the braided fiber composite element will then surround the core element and the axis of curvature of the concavity, in a loop-shaped fashion.
According to a particularly preferred embodiment, the at least one concavely constructed face of the core element is adjoined by an interior element for forming the corresponding passage opening. The shape (for example, the radius of curvature) of the interior element is adapted to the shape (or the curvature) of the at least one concavely constructed face, and the core element and the outer circumferential surface of the interior element are enclosed in a loop-type fashion by the braided fiber composite element. The shape-adapted embodiment of the concavely constructed face and the corresponding interior element ensures a continuous flush transition, without gaps, slots, air pockets or cavities. The latter would lead to unwanted and disadvantageous accumulations of resin when impregnating the semi-finished fiber composite product. In addition, the use of such an interior ring permits a better introduction of forces into the core element. Furthermore, the interior ring can be machined in a simple manner, for example, by milling a fitted bore for a bearing to be accommodated.
The interior element advantageously consists of a fiber composite material which is laid or arranged, for example, in the form of prepreg layers or dry semi-finished products, around a corresponding holding element, as described in greater detail hereinafter. However, the interior element may also be produced by means of a winding process. Particularly preferably, a braided fiber composite tube section consisting of carbon, glass and/or aramid fibers is used, which can also be produced by means of the above-mentioned circular braiding technique.
The core element designed for the absorption of compression forces preferably has the shape of a rectangular parallelepiped and has corresponding longitudinal and transverse sides. In the case of concave curved faces, the axis of curvature of the concavity preferably extends perpendicular to the longitudinal sides. The axis of curvature of the concavity may also have a different orientation (for example, perpendicular to the transverse sides of the core element). Naturally, cubic-shaped, cylindrical, conical, pyramidal or core elements of another geometrical shape can also be used. The respective faces may have a recessed form either on one side or on both sides. The two faces may naturally also be differently formed.
Since the compression forces to be absorbed by the core element are low in comparison to the tensile forces to be absorbed, the core element may have a sandwich-type structure in order to save weight. The core element advantageously consists of a foam core which is surrounded in a braided manner with the formation of individual braided layers. The foam core may consist of PU foam or other conventional foamed materials.
The braided layers advantageously enclose the cross-section of the foam core in a concentric or onion-type manner. This is achieved in that the braiding takes place around the foam core in its longitudinal direction, so that the transverse and longitudinal sides are covered by braided layers.
Particularly preferably, the braiding around the foam core takes place unidirectionally at braiding angles of ±45°. This can take place, for example, by means of the initially mentioned circular braiding technique, by which, during the braiding of a first layer, carbon, glass and/or aramid fibers are interlaced as reinforcing threads at an angle of +45°, and Grilon® and/or glass fibers are interlaced as supporting thread at an angle of −45°. The orientation of the braiding will then change when the next layer is braided; that is, the supporting threads have an orientation of +45° and the reinforcing threads have an orientation of −45°. Other braiding angles may of course also be used.
Fiber composite fabrics covering at least the longitudinal sides of the core element may be arranged between the individual braided layers. These fiber composite fabrics also consist, for example, of carbon, glass and/or aramid fibers and preferably have an orientation of 0°, 45° or 90°. The fiber composite fabrics have the effect that high compression forces can be better absorbed, and is therefore used as reinforcement.
As an alternative, the fiber composite component according to the invention may have a core element which consists of correspondingly tailored and stacked prepreg layers or dry semi-finished products and can be produced quasi manually. In this case, layers with 0°, 45°, or 90° orientations are preferred. Such a core element can be produced in a simple manner (for example, also in multi-component production) and has an extremely low weight.
In order to ensure an optimal absorption of the tensile forces by the fiber composite element surrounding the core element as well as the at least one passage opening or the at least one interior element in a loop-type manner, the braided fiber composite element is braided in a direction perpendicular to the longitudinal direction of the core element or, depending on the application, in a direction perpendicular to the transverse direction of the core element, preferably at braiding angles of ±85° with respect to these directions. Generally, a steep braiding angle, that is, a greater angle with respect to the braiding direction, is advantageous in order to be able to better absorb tensions in the braiding fibers. The braiding fibers then essentially have the orientation of the load and are optimally utilized. For the braiding, carbon, glass and/or aramid fibers are again preferably used, depending on the requirements, as required, here also, glass fiber and/or Grilon® supporting threads being applied.
For producing fiber composite components according to the invention, several core elements are required, each having at least one concavely constructed face, as well as at least one holding element, with the at least one concavely constructed face being adapted to the shape of the corresponding holding element. In a first step, several core elements are arranged in a stack such that, in each case, the at least one concavely constructed face is applied by resting against the corresponding holding element. This arrangement of stacked core elements and the at least one holding element is then fixed, as required, by means of spacers. In another step, the fixed arrangement is, for example, clamped into a circular braiding machine and a circular braiding takes place for forming a fiber composite element enclosing the core elements and the at least one holding element in a loop-type manner. Then the circularly braided arrangement is infiltrated and hardened. Subsequently, the at least one holding element is removed from the mold (if required, by means of chilling), and finally the circularly braided stacked core elements are separated from one another by means of cutting, sawing or milling in order to obtain separate fiber composite components which each have a core element.
In this manner, several fiber composite components can be produced simultaneously and can be separated from one another in the last processing step, by means of cutting, sawing or milling. This represents a particularly effective production method because other techniques, as a rule, allow only the production of a single component at a time.
According to a preferred method, the at least one holding element is covered with fiber composite material before the stacking of the core elements, so that, when stacked, the in each case at least one concavely constructed face rests against the fiber composite material of the corresponding holding element.
For this purpose, a braided fiber composite tube is preferably used, which is pulled over the at least one holding element in the manner of a stocking, before the stacking or placing of the core elements. This fiber composite tube can, in turn, be produced by means of the initially mentioned circular braiding technique by using carbon, glass and/or aramid fibers (as required, with corresponding supporting threads). However, the holding element can also be provided with the fiber composite material by the winding around or placing around of corresponding fibers or fabrics. In some cases, it is advantageous to compact (that is, condense, and/or harden) the fiber composite material after the application to the holding element.
To reduce weight, as mentioned above, the core element is formed by braiding individual layers around a foam core which is subsequently precompacted. The braiding around the foam core typically takes place in its longitudinal direction at braiding angles of ±45° by using carbon, glass and/or aramid fibers (and, as required, glass fiber and/or Grilon® supporting threads). In this case, fiber composite fabrics of carbon, glass and/or aramid fibers can be arranged between the individual braided layers, preferably with an orientation of 0°, 45°, or 90°.
In addition to the core elements produced as described above and consisting of circularly braided foam cores, additional core elements can be produced, where the foam core is surrounded by a separating foil before the circular braiding. This separating foil may, for example, be a Teflon® foil which is provided for facilitating a later separation of the individual fiber composite components. In the following, core elements with a separating foil surrounding the foam core will be called “loss cores”.
The above-described core elements, including the so-called loss cores, can be produced as quasi continuous material and can subsequently be cut to the required measurement and precompacted. Then the at least one face is preferably machined by means of ultrasonic cutters. This technique results in better cut surfaces (for example, in comparison to blanking methods) since the foam core is locally compressed in the case of the latter. Because of the production of continuous material, the efficiency of the production process is increased more advantageously because a braiding around quasi several core elements takes place in one processing step.
The stack arrangement of the core elements can take place in different fashions. Either core elements whose foam cores are not surrounded by a separating foil are arranged alternately adjacent loss cores such that adjacent longitudinal sides of core elements and loss elements are each in contact with one another, or only core elements are used without the separating foil surrounding the foam core, in which case respective separating foils are then arranged between adjacent longitudinal sides of the core elements. The separating foil has the purpose of finally being able to better separate the individual fiber composite components which each contain a core element.
After the stacking or arranging of the core elements (or of the core elements and loss cores) by means of the at least one holding element, this arrangement is fixed, as required, by means of spacers in order to then braid around it, preferably by means of the circular braiding technique. For this purpose, the arrangement is disposed in a circular braiding machine such that the braiding takes place in the direction of the longitudinal axis of the at least one holding element. A steep braiding angle is advantageous in this case, so that the tensions can be better absorbed in the braiding fibers. Preferably a braiding angle of ±85° with respect to the braiding direction or longitudinal axis of the at least one holding element is used. Naturally other braiding angles can also be used. For the circular braiding, typically carbon, glass and/or aramid fibers can again be used. In this case the waviness of the braiding can be reduced in that additionally supporting threads are used consisting of glass and/or Grilon® fibers.
The thus circularly braided arrangement is then infiltrated typically by means of resin vacuum infiltration processes (for example, resin transfer molding or RTM, or vacuum assisted process or VAP) and is hardened. The hardening temperatures vary according to the used resin system. Typically, the hardening temperatures are in a range of from 100-200° C. Then the at least one holding element is removed, as required, by means of chilling.
Finally, the arrangement for separating the individual fiber composite components differs, depending on whether only core elements (without a separating foil surrounding the foam core) were used, or core elements and loss cores were arranged alternately in a stack. In the former case, the cutting, sawing or milling takes place along the separating foils arranged between the individual core elements. In the case of alternatingly stacked core elements and the loss core, the loss core is “sacrificed” in each case, because it is cut through in its longitudinal direction.
The fiber composite component according to the invention may be used, for example, as a joining element, compression-tension strut, rear link or transverse link in aerospace engineering (for example, in airplanes or helicopters) or in automotive engineering.
A particularly preferred application of the fiber composite component according to the invention is the bearing of landing flaps, particularly for the rear bearing of airplane landing flaps.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.